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The Nature, Testing, and Variables Influencing THE NATURE, TESTING, AND VARIABLES INFLUENCING FIXATION DISPARITY; ROLE OF THE FUSION LOCK 1 by Leland W. Carr with Dr. James J. Saladin I ... *I Tl.e N.-1• .._1 Tufi"), A.l IJ.,.;.,l/.s I.{l..•• :? F1*'/:·.., /);,,_,. ; ~ R.lc. OF 'Tlc. ~="••1•.. C. •• l( I Introduction A fixation disparity is a small angular measurement of the mis­ alignment of the two eyes which can occur while still permitting single, fused binocular vision. It represents a small error in the aiming of the eyes which occurs without diplopia being detected. Fixation disparity is allowed because of the slight "slippage" pro­ vided to the fusional system through the existence of Panum's fusional areas. So long as binocular alignment is precise enough to place the two retinal images of a single object within corre­ sponding Panum's areas, the final perception is likely to be single and fused. Sensory fusion thus occurs in spite of a small error in motor fusion. A slight muscle imbalance is the rule, rather than the excep­ tion in individuals even with normal assymptomatic binocular systems. It is rare that all twelve extraocular muscles are precisely bal­ anced in their agonist-antagonist relationships, and thus the inate drive to achieve single vision requires a fusional effort to over­ come existing imbalance. When binocularity is dissociated (as with a covertest) the fusional drive is interrupted and the eyes deviate out of alignment under the influence of the muscle imbalance. This deviation under dissociation is referred to as the heterophoria. When both eyes are permitted to view without dissociative conditions the fusional drive to achieve single binocular vision pulls the eyes toward alignment in opposition to the phoric "stress" operating to deviate the alignment. Because of Panum's areas, motor fusion need only achieve I 2 alignment within the corresponding regions, rather than obtain pin­ point- precise binocular alignment, and fusion will occur. Thus fixation disparity measures the amount of misalignment occurring under fused (though "stressed") binocular viewing situations. It is the intent of this paper to briefly summarize the his­ tory and techniques of measuring fixation disparity, and to then examine one of the major variables of the measurement technique - the size of the fusional stimulus (fusional "clue", fusional "lock", fusional "ring") and its impact on the measurements obtained. History The earliest work on fixation disparity came from observations by Hofmann and Bielschowsky with the synoptophore which revealed that exo f.d. increased as convergence demand increased. Ames and Glidden also worked with haploscopic targetsJ consist­ ing of central large block E l e tters with peripheral horizontal and vertical dots. They found that superimposion of the peripheral dots could still leave the E slightly doubled indicating a slight mis­ alignment of the lines of sight, which they called an "inexactness of fixation associated with a heterophoria." The majority of the f.d. experimentation was done under the supervision of Ogle. (I) His experiments show that patients with a phoria (in which the eyes have a tendency to turn inwards or out­ wards) will actually under - or overconverge by a small amount, thus resulting in disparate images falling on the two retinas. These disparate images will be fused, however, and the subject will not be aware of disparity. I 3 Through trial-and-error experimentation Ogle found that ver­ tical vernier lines would provide the most accurate means of investi­ gating these small deviations in alignment of the eyes. To measure fixation disparity most accurately requires that the vernier lines be seen foveally where accuity is at its finest, and that they be seen bi-ocularly with one eye viewing one line while the other eye views the other line. This means that central­ region fusional details must be eliminated, and thus periphera~ fusion clues are utilized while central target lines are used to measure misalignment under fusing conditions. In a patient with normal ocuities in both eyes, a displace­ ment of + 1.0 minute of arc can be detected in vernier lines. An interesting sidelite is that the test of fixation disparity using small centrally located vernier lines can indicate a foveal suppression if one line is not visible while all of the peripheral clues, and the other line, are visible. Ogle determined that the fixation disparity was evidence that an oculomotor muscle imbalance exerts a continuous effort to devi- ate the eyes to the heterophoric position, and that it was an entity called "fusional convergence" which acted in opposition to the phoria to direct the eyes to "near-enough" alignment that diplopia was avoided. A small error in precise convergence (or divergence) occurs most often in the same direction as the phoria, thus causing a sin­ gle fixation point to be imaged onto slightly noncorresponding (dis­ parate) points on the two retinas; the amount of this disparity Ogle called the "fixation disparity". A f.d. cannot be larger than the limits of corresponding Panum's fusional areas or diplopia (or / 4 suppression) will occur. Therefore, Ogle tells us that a fixation disparity is evidence that fusion occurs without full compensation of the phoria, and ~;;. represents a manifest deviation of the visual axis in spite of sue- cessful fusion. The size of the fixation disparity depends pri- marily upon the size of the muscle imbalance (the amount of the phoria), but Ogle also felt that it must depend upon the stimulus to fusion. According to Ogle, as ·larger fusional clues stimulate more peripheral areas of the retina where Panum ' s areas are larger, he would expect fusion to occur with less precise binocular alignment. With more "slop" permitted in alignment, he should expect to have larger fixation disparities. Ogle also felt that the fixation disparity will increase as the effort to maintain fusion is incr eased. As one works harder to achieve fusion (for examp l e near t he vergence limits) the f.d. will increase in size . Clinical Measurement of Fixation Disparity Measurement of fixation disparity becomes a more meaningful indicator of a healthy binocular system when it is checked under various levels of fusional "stress". Prisms are used to alter the demand on the convergence/divergence mechanism, shifting the fusional demand, and changing the innervation to the extraocular muscles. Fixation disparity is measured under various conditions of fusional stress. (II) Base-in prism will create an innervational esophoria because the stimulus to accommodation will be nearer than the stimulus to I 5 fusion, requiring negative fusional innervation in order to see clearly and singly. Accommodative convergence is acting to turn the eyes inward while fusional drive acts to aim the eyes outward (innervational esophoria!) Base-out prism acts in a reverse manner to create the inner­ vational situation of an exophoria. As base- in prism is increased toward the limits of the nega­ tive vergence, then the fixation disparity will most often increase in the eso disparity direction (an underdivergence), showing an increasing esophoric oculomotor imbalance. Once the base-in break point is reached diplopia occurs and the eyes shift to their natural phoric position. Again the reverse situation is found for increasing base- out prism. Host often this will give an increasing exo-disparity up to the convergence breakpoint . Fixation disparities are quite s mall angles, measured in minutes of arc. They are interpreted based upon their curve when plotted on a two coordinate graph. The abscissa lists the base-in and base- out prism power placed before the eyes, while the ordinate lists the eso or exo disparity measured. That value of prism power before· the eyes when the f. d. is found to be zero is called the "associated horizontal phoria". It indicates the amount of muscle imbalance when the eyes are precisely aligned upon the fixation target (notice the assumption of normal retinal correspondence has been made!) The term "associated" phoria is used because the binocularity has not been drastically ''disas­ sociated" as by maddox rod or vertical prism used to determine the I 6 "disassociated phoria". The point on the fixation disparity graph where the oculomotor imbalance is zero (where the curve crosses the abscissa) indicates the prism value of the associated phoria. It indicates the relative rest position of the ext~aocular muscles for the subject for that observation distance; when fusion is main- tained. (III) Accuracy of f.d. measurement by the use of a two-line nonius alignment is acuity dependant, and subjects with 20/20 acuity in each eye are able to discriminate 1' of arc or even smaller dis­ parities. Sensitivity is altered only slightly if both eyes are equally blurred, but is more significantly decreased if the two acuities are significantly different, and is quite variable in sub­ jects with larger oculomotor imbalances. The amount of f.d. tends to vary directly as the size of the phoria. As mentioned previously, Ogle considered the role of "periph­ eralness" of the fusion-lock in determinations of f.d. by the ver­ nier lines method. He expected more retinal slippage (more misalign­ ment) to be tolerated by the system before diplopia occurred, when using a more peripheral fusion target. The small Panum's areas associated with the macular area could not tolerate much disparity, or misalignment, before diplopia would occur. Thus Ogle predicted larger f.d. for a more peripheral target for fusion. Ogle's preliminary investigations did find larger fixation dis­ parities when the fusion-lock portion of the test target was made larger (therefore made more peripheral on the retina). One major finding of this initial work was that the associated phoria was not changed by the size of the fusion-lock.
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